3d-printing device and process for producing an object with use of a 3d-printing device

ABSTRACT

Improved 3D printed articles are produced in a 3D printing apparatus by controlling the apparatus in response to the measurement of the size, geometry, and/or weight of print materials exiting the discharge device of the apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase of PCT Appln. No. PCT/EP2016/059094 filed Apr. 22, 2016 and claims priority to PCT/EP2015/080742, filed Dec. 21, 2015, the disclosures of which are incorporated in their entirety by reference herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates to a method of manufacturing an object using a 3D printing device having at least one printhead having at least one discharge device, wherein the discharge device is set up to place print materials at target positions in order to additively manufacture the object. Further aspects of the invention relate to a 3D printing device configured to conduct the method.

2. Description of the Related Art

A multitude of different additive manufacturing methods are known from the prior art for production of prototypes, short runs or individual articles. What is common to these methods, also referred to as 3D printing, is that an article or object is manufactured directly on the basis of a computer model. Advantageously, it is thus possible to manufacture customer-specific components inexpensively and easily. For production of an object, for example, a powder is selectively consolidated by applying a hardener, with application of the hardener to the powder in a pattern dependent on the object to be manufactured. Further methods include laser sintering, in which powder is consolidated by melting with a laser in the desired form according to a defined pattern, and fused filament fabrication, in which an object is produced layer by layer from a fusible plastic. There are likewise known methods in which liquid is released dropwise with nozzles and cured, for example, by action of UV radiation.

DE 10 2011 106 614 A1 discloses a method and a device for production of a three-dimensional article, wherein the article is constructed from a consolidatable material such as silicone, which is either in the liquid phase in its original state or can be liquefied. The liquid material is discharged in the form of droplets and positioned on an object carrier. By means of variable alignment, positioning and inclination of a printing stage or of the object carrier, special 3D prints of overhangs and self supporting elements are implemented. The object to be printed here is aligned by multiaxial actuators such that the printing unit can always place the print voxel vertically on the printing plane.

DE 10 2012 000 664 A1 discloses a device for manufacture of three-dimensional objects having a carrier and an extruder for release of a print material and having a drive system, wherein the carrier and the extruder are movable relative to one another in three directions of movement by means of the drive system, in order to manufacture the three-dimensional object. To improve adjustability, multiple drive motors are provided in at least one of the directions of movement.

EP 1 886 793 A1 describes various aspects relating to influencing the droplet size and influencing the flight path of print droplets. In addition, the distance from the print nozzle to the printing plane is chosen so as to result in optimal detachment of the printing drops.

DE 10 2013 003 167 A1 relates to a further method of manufacturing a three-dimensional article by additive manufacturing. On deployment, structurally different regions of the article are manufactured, with manufacturing of spatial structures according to selected configuration criteria on deployment in the different regions.

DE 10 2015 110 342 A1 describes yet a further method of printing conductive elements onto any desired bodies. The accuracy of droplet positioning is improved by optimizing the size of the distance between the nozzle tip and the substrate.

However, the devices known from the prior art have technical detects which affect the quality of the printed parts. The quality of the objects achievable by the additive methods known from the prior art does not reach the constant quality of comparable objects produced by means of injection molding. Nor is it possible by the known methods to ensure uniform quality of the end product, as is indispensable for the industrial use of the objects produced.

Troublesome factors are in particular misprints which arise owing to changes in the material discharge from the print nozzle, or misprints having defects such as air bubbles. Air bubbles of this kind arise, for example, in the case of unplaced print materials.

One object of the invention is that of providing an improved method for additive manufacture of objects and a corresponding device with which objects of high quality, for example in relation to surface and trueness to shape, are producible. A further object of the invention is that of providing a method by which a uniform quality of the objects produced can be ensured.

SUMMARY OF THE INVENTION

The invention is directed to a method of producing an object using a 3D printing device. The 3D printing device has at least one printhead having at least one discharge device, wherein the discharge device is configured to place print materials at target positions in order to additively manufacture the object. According to the invention, print materials that are exiting or have exited from the discharge device are detected and/or geometrically measured, in the region of the discharge device. The print material may be material for manufacture of a permanent component, especially silicone, or may be material which is required for temporarily manufactured parts or regions, especially support material in the form of polyethylene glycol (PEG), for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a working example of a method of actuating a discharge device,

FIG. 2 illustrates a schematic setup of a 3D printing device in one embodiment of the invention,

FIG. 3 illustrates a side view of a strand discharge and a strand measurement in one embodiment of the invention,

FIG. 4 illustrates a top view of the strand measurement as per FIG. 3,

FIG. 5 illustrates a side view of a droplet measurement in one embodiment of the invention,

FIG. 6 illustrates a side view of a printhead and optical measurement unit arranged in a holder in one embodiment of the invention,

FIG. 7 illustrates a side view of a printhead with an emitter and a receiver in one embodiment of the invention and

FIG. 8 illustrates a side view of a printhead with an emitter/receiver unit in one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The print material may be material for manufacture of a permanent component, especially silicone, or may be material which is required for temporarily manufactured parts or regions, especially support material in the form of polyethylene glycol (PEG), for example.

The discharge device is set up to release print materials in the form of individual isolated droplets, as a series of droplets or in the form of a strand in the direction of the discharge axis. Flowing transitions between these forms are possible. In the context of this description, a droplet of a print material discharged from the discharge device and placed on the baseplate or on the object is referred to as a voxel. A strand refers to both discharged and as yet unplaced print material and to placed print material. A placed print material is understood to mean either a voxel or a strand.

The print materials are detected and/or geometrically measured in the region of the discharge device, and so the measurement allows conclusions as to the state of the discharge device. This means that the print materials are detected and/or geometrically measured white they are exiting or after they have exited and before they have been placed. In the case of droplets, this means that they are detected and/or geometrically measured during flight. The measurement of the print materials in the region of the discharge device can be supplemented by further measurements on the print materials after they have been placed.

Geometric measurement refers in a broad sense to the gaining of knowledge as to the form or shape of the print material that has exited.

In one configuration of the method, print errors are detected on the basis of the geometry measured. For example, it is possible to ascertain whether print material in the form of a droplet, a series of droplets or a strand has exited at all. More particularly, the detection and/or geometric measurement can thus enable the determination of erroneously unplaced print materials. Erroneously unplaced print materials can be an indication, for example, of wear to pressure rams or to blockage of the discharge device. If this is detected to an intolerable degree, provision may be made, for example, to clean the discharge device or to issue maintenance advice to the user.

In addition, the droplets, droplet series and strands can be more accurately characterized by measuring the droplet form, droplet series form or strand form. It is possible here to determine, for example, the diameter of the droplet cross section and of the strand. In addition, the volume of the droplet can also be measured, by means of which, given known density of the print material, it is also possible to determine the weight of the droplet. In addition, it is, also possible to determine the degree of rotational symmetry of the droplet or strand. In the case of a droplet, it is possible to determine the deviation from a defined droplet geometry. In the case of a strand, it is possible to determine the deviation from a defined strand geometry, for example from cylinder geometry. As a result, for example, it is also possible to detect and correct a wrong discharge height or a wrong movement speed of the printhead in the printing of the strands, which lead to deformation of the discharge strand from the discharge device.

The geometric measurement may additionally or alternatively also include a determination of unwanted breakoff characteristics of the print materials, especially a determination of stringing. In the printing of voxels, shortly before the detachment of the droplet from the discharge device, there arises a shape in which the droplet is virtually spherical on one side, but tapers to a point on the other side. After the detachment, the tip can become smaller or disappear entirely. Under certain conditions, for example too low a reactant temperature, partly crosslinked material or under suboptimal conditions of the discharge device, especially in the event of a wrong ram advance speed, wrong ram withdrawal speed, wrong opening times of the nozzle or wrong ram stroke, however, the tip does not disappear but widens out to form a thread which can be so marked that it connects the droplet to the subsequent droplet. This effect is referred to as stringing. Stringing can also exist in the case of a strand, for example when strands are discontinued from layer to layer and commenced again. If stringing is detected to an intolerable degree, provision may be made, for example, to clean the discharge device.

In a preferred embodiment, the weight of the print materials discharged from the discharge device is also measured. During the printing process, it is thus possible for a continuous mass balance of the printed object to be established. The weight of the print material determined by means of volume and density can be compared with the measured weight. Thus, recognition of any introduction of foreign bodies into the print material during the printing operation is achievable. Foreign bodies may, for example, be floating particles from the environment or else come from solidified silicone deposits or from condensed water in the discharge device.

A different geometry or an erroneously unplaced print material can indicate problems with the discharge device, for example a blocked nozzle or an air bubble in the feed of the print material to the printhead. The detection or measured geometry of the print materials that have exited from the discharge device, in one embodiment of the invention, is therefore used to trigger automatic cleaning of the printhead. The cleaning serves for quality assurance since only in the case of a clean nozzle is the desired voxel, i.e. strand geometry, assured. For this purpose, the 3D printing device has a cleaning station which enables automatic cleaning.

If an erroneously unplaced print material is recognized, this is recorded together with the position at which the print material should be placed. One embodiment of the invention envisages reprinting of the erroneously unplaced print materials. As a result, less reject material resulting from erroneously printed objects is to be expected. In order to reprint the unplaced print materials, the printhead, for example after cleaning thereof, is moved back to the recorded position and the discharge of the print material is repeated. The erroneously unplaced print materials can subsequently also be placed by controlled multiple prints in subsequent layers.

Preferably, the reprinting of the unplaced print materials precedes a method step of curing. The reprinting of the unplaced print materials prior to the curing has the advantage that the reprinted print materials can still bind to the uncured print materials, for example to the placed adjacent print materials, where the term “adjacent” may relate, for example, to print materials in the same layer, or, according to the curing strategy, also to print materials in multiple layers. For this purpose, the print material used is still free-flowing after application, such that the print materials placed can merge with one another, resulting in a smooth transition between the print materials placed.

Alternatively or additionally to the reprinting of the unplaced print materials, error messages or warning messages may be issued. The aging of the discharge device can be recorded in the form of error messages. Warning messages may include maintenance advice for the user, for example point out that cleaning is necessary.

The results of the measurement of the geometry of the print materials that have exited can be provided to a user of the 3D printing device for other purposes as well, for example on a screen and/or on a data carrier. The user is thus put in a position to be able to react appropriately to any change in the discharge of the print material.

Some recognized deviations from standard geometry that occur can be compensated for by automatic corrections in the device control system. In one embodiment of the method, the print outcome, i.e. the desired geometry of the exiting print materials, is under automatic closed-loop control on the basis of the measurement results. In the context of the present disclosure, closed-loop control, following standard terminology, is understood to mean an automatic change in an operating parameter to maintain a measurement parameter, here a uniform geometry. In simplified terms, this can also be described as follows: If the geometry differs too significantly, one or more of the parameters mentioned hereinafter are readjusted until the geometry is again as desired. If this is not possible, the 3D printing operation can be stopped. The latter will be the case especially in the event of changes in print quality owing to wear to the discharge device or in the event of erroneous or missing print material.

The operating parameters of the 3D printing device that are capable of affecting the geometry of the print materials that have exited from the discharge device include the movement speed of the printhead. An increase in the movement speed of the printhead in the case of strands can lead to a reduction in the strand cross section in movement direction, and a decrease in the movement speed of the printhead correspondingly to an enlargement of the strand cross section in movement direction.

The operating parameters of the 3D printing device that are capable of affecting the geometry of the print materials that have exited from the discharge device also include the discharge height or application height above the baseplate on which the object is disposed. An increase in the discharge height above the baseplate in the case of strands can lead to inexact positioning and in the case of voxels to trajectory errors; a reduction in the discharge height above the baseplate in the case of strands can lead to lateral deformation and in the case of voxels to unclean droplet breakoff characteristics up to and including blocking of the nozzle, and to stringing phenomena. More particularly, the immersion of the discharge device into the placed print material is undesirable in most cases.

The operating parameters of the 3D printing device that are capable of affecting the geometry of the print materials that have exited from the discharge device additionally also include the physical pressure that prevails in the discharge device, to which the print material is subjected. The expression “physical pressure” is used to distinguish the expression from “printing”. An increase in the physical pressure that prevails in the discharge device leads to elevated material discharge in the case of strands and in the case of voxels. A reduction in the physical pressure that prevails in the discharge device leads to a reduction in material discharge in the case of strands and in the case of voxels.

The same applies to the physical supply pressure in the material reservoir.

The operating parameters of the 3D printing device that are capable of affecting the geometry of the droplets that have exited from the discharge device further include the print frequency, or discharge rate of the droplets or voxels. An increase in the print frequency can lead to a reduction in volume, and a decrease in the print frequency correspondingly to an increase in volume. The print frequency is further preferably adjusted in combination with the movement speed of the printhead.

Further parameters of the jetting nozzles that affect the voxel size are the ram advance speed, ram withdrawal speed, opening times of the jetting valve and the ram stroke. Closed-loop control of the geometry of the print materials that have exited from the discharge device by variation of these parameters is therefore likewise proposed. Ram shapes and nozzle combinations also affect the voxel size, but these parameters often cannot be altered during the, printing.

Further parameters of the dispenser that affect the strand size and print accuracy are the flow rate, feeder speed, the supply pressure in the material reservoir and the material contraction characteristics at the end of a line. Closed-loop control of the geometry of the print materials that have exited from the discharge device by variation of these parameters is therefore likewise proposed.

Measurement Methods

The measurement is preferably effected from one side, i.e. at right angles to the discharge direction of the print materials, such that, for example, a droplet that has exited is detectable with its spherical section and tipped section. In the case of a measurement by means of a camera, its field of view is directed correspondingly. In the case of a luminance measurement, the light barrier is arranged correspondingly.

If voxels are being printed, in order to be able to determine stringing and droplet, geometry, preference is given to a light beam broader than the maximum possible droplet diameter. In the case of strands, measurement with a light beam broader than the strand diameter is meaningful. In terms of cross section, the light beam is, for example, 0.3 mm in diameter with round geometry, and 0.1 mm×0.3 mm in the case of quadrangular geometry.

The measurement is preferably effected as close as possible to the discharge device, in order that, for example, the ascertained properties of the droplets and strands permit sufficient conclusions as to the state of the discharge device and are s not distorted by environmental effects such as floating particles. Thus, the distance of the light beam from the discharge device is preferably below 1 cm, preferably below 1 mm, especially preferably below 0.1 mm.

The geometric measurement can be effected with the aid of cameras which cover the region of the exit orifice of the discharge device. The image frequency of the camera is matched to the jetting frequency such that every droplet can be seen in at least one camera image. The computer-assisted evaluation of the camera images in that case includes a determination of the outline of the droplet and a comparison with an ideal droplet. In the case of high print frequencies, preference is given to using high-speed cameras with several hundred to several million images per second.

Preferably, the geometric measurement is effected, however, by means of at least one luminance measurement in transmitted light operation or in reflected light operation. This involves transmission of a light beam by an optical emitter, which is detected by an optical receiver. This embodiment of the invention is in many cases more favorable than the use of a high-speed camera and corresponding image processing.

In transmitted light operation, the receiver is arranged facing or opposite the receiver. If a print material crosses the light beam, it generates a shadow behind it owing to reflection and absorption, i.e. a measurable decrease in the luminance hitting the receiver The received signal against time is also referred to as “function of shadowing”. The presence of print material is concluded from the decrease in the luminance hitting the receiver. More exact analysis of the received signal against time permits conclusions as to the shape of the print material that has exited.

In reflected light operation, receiver and emitter are arranged at an angle of 0 to 90° relative to one another. At an angle of 0°, the emitter is simultaneously the receiver, which in the present disclosure is also referred to as emitter/receiver unit. The measurement principle here is as follows: If a print material crosses the light beam, it generates a measurable signal owing to reflection. The receiver measures the reflected fraction of the light emitted. The presence of print material in the light path is concluded from the increase in the luminance hitting the receiver. More exact analysis of the received signal against time permits conclusions as to the shape thereof.

If multiple luminance measurements are conducted, for example by multiple emitters and/or receivers, it is additionally possible to determine a different position or a non-straight flight path of the droplet or a different location of the extruded strand. This can also be determined in the case of V arrangement (angle greater than 0° and less than 90°) of emitter and receiver.

In the evaluation of the received signal, a comparison of the signal received over time with a reference signal is conducted. The reference signal may come here from a database in which signals originating from simulations or test runs are stored. Alternatively, the reference signal is recorded in a calibration of the 3D printing device or in each case on commencement of the printing of the object or a layer of an object. In these cases, it is advantageously possible to react to the change in the material discharge over time, and the process can proceed independently of the operating parameters established.

The received signal in a luminance measurement corresponds in physical terms to convolution of the droplet form with the sensor geometry, i.e. in one-dimensional form

${{FS}(t)} = {{\left( {g*f_{emp}} \right)(t)} = {\frac{1}{T}{\int_{a}^{a + T}{{{g(\tau)} \cdot {f_{emp}\left( {t - \tau} \right)}}d\; \tau}}}}$

where f_(emp) represents the droplet form and g the sensor geometry, and where t denotes the time, T denotes the period and a denotes a constant for selection of the period in question.

The comparison of the received signal with the reference signal can be made using an electrical engineering filter or in a computer-assisted manner with the aid of known mathematical methods. An electrical engineering filter used may, for example, firstly be an optimal filter for comparing the received signal with the reference signal. Thereafter, a tolerance band, around the reference signal is fixed, and it is determined whether the received signal is within the tolerance band. In computer-assisted comparison, it is possible, for example, first to use a known best-fit algorithm in order to model the received signal and the reference signal successively in time. The degree of the deviation can then be determined, for example, by means of breakdown of the time signals into Fourier coefficients or the like and by comparison thereof. Alternatively or additionally, for this purpose, the degree of deviation can be determined by means of forming the integral or the sum total over the squared difference between the time signals. The determination of the deviation can also include the examination of the derivatives against time or the integral of the signals.

In the case of print materials in the form of voxels, the geometric measurement of the voxels is effected with a scanning rate of at least five times, preferably at least, ten times, the print frequency of the voxels. This ensures that sufficient data are available such that the geometric measurement of the exiting print materials is meaningful.

Print Material

The print material used is preferably a material which is in a free-flowing form at least during processing and can be cured after discharge. The subsequent curability, in the case that a misprint is detected, means that a process, for example cleaning of the printhead followed by reprinting of erroneously unplaced print materials, can be conducted, in which case the uncrosslinked materials remain free-flowing until they have been cured, such that the subsequently placed print materials can still become bonded to the print materials placed prior to the cleaning.

It is preferably the case that curing of the print materials is effected by means of radiation or by thermal means, more preferably in a location-selective manner or over the full area by means of radiation or thermal means. Preference is thus given to using, in the process proposed, print materials which, after being placed, can be cured via action of radiation or heat.

Location-selective exposure is understood to mean that the heat or radiation source is arranged in a movable manner relative to the baseplate or acts only on selected regions of the object. An areal exposure is understood to mean that the heat or radiation source acts over the entire object or an entire material layer of the object.

For example, in the case of the process proposed, a print material which can be cured via action of actinic radiation is used, preferably by action of UV/VIS radiation. UV radiation or UV light has a wavelength in the range from 100 nm to 380 nm, while visible light (VIS radiation) has a wavelength in the range from 380 to 780 nm. Preferably, the action of the UV/VIS radiation on the print material is effected via an exposure unit.

In the case of print materials that cure by thermal means, it is possible to use an infrared source (IR) in order to conduct a location-selective or areal heat treatment.

In the method of the invention, the print materials used are more preferably silicone rubber materials that crosslink via UV/VIS-induced addition reaction. UV/VIS-induced crosslinking has advantages over thermal crosslinking. Firstly, intensity, action time and action site, of the UV/VIS radiation can be judged accurately, while heating of the discharged print material (and subsequent cooling thereof) is always delayed owing to the relatively low thermal conductivity. Owing to the intrinsically very high coefficient of thermal expansion of the silicones, the temperature gradients that inevitably exist in thermal crosslinking lead to mechanical stresses which adversely affect the trueness to scale of the object formed, which in the extreme case can lead to unacceptable distortions in shape.

UV/VIS-induced addition-crosslinking silicone rubber materials are described, for example, in DE 10 2008 000 156 A1, DE 10 2008 043 316 A1, DE 10 2009 002 231 A1, DE 10 2009 027 486 A1, DE 10 2010 043 149 A1 and WO 2009/027133 A2. The crosslinking occurs through UV/VIS-induced activation of a light-sensitive hydrosilylation catalyst, preference being given to complexes of platinum. The technical literature describes numerous light-sensitive platinum catalysts which are largely inactive with, exclusion of light and can be converted by irradiation with light having a wavelength of 250-500 nm to platinum catalysts that are active at room temperature. Examples of these are (η-diolefin)(σ-aryl)platinum complexes (EP 0 122 008 A1; EP 0 561 919 81), Pt(II)-β-diketonate complexes (EP 0 398 701 B1) and (η5-cyclopentadienyl)tri(σ-alkyl)platinum(IV) complexes (EP 0 146 307 81, EP 0 358 452 B1, EP 0 561 893 B1). Particular preference is given to MeCpPtMe₃ and the complexes that derive therefrom through substitution of the groups present on the platinum, as described, for example, in EP 1 050 538 B1 and EP 1 803 728 B1. The print materials which crosslink in a UV/VIS-induced manner can be formulated in one- or multicomponent form.

The rate of the UV/VIS-induced addition crosslinking depends on numerous factors, especially on the nature and concentration of the platinum catalyst, on the intensity, wavelength and duration of action of the UV/VIS radiation, the transparency, reflectivity, layer thickness and composition of the silicone rubber material, and the temperature.

The platinum catalyst is preferably used in a catalytically sufficient amount, so as to enable sufficiently rapid crosslinking at room temperature. Preference is given to using 0.1 to 500 ppm by weight of the catalyst based on the content of Pt metal relative to the overall silicone rubber material, preferably 0.5 to 200 ppm by weight, more preferably 1 to 50 ppm by weight.

For the curing of the silicone rubber material that undergoes addition crosslinking in a UV/VIS-induced manner, preference is given to using light of wavelength 240 to 500 nm, more preferably 250 to 400 nm, yet more preferably 350 to 400 nm, and most preferably 365 nm. In order to achieve rapid crosslinking, which shall be understood to mean a crosslinking time at room temperature of less than 20 min, preferably less than 10 min, more preferably less than 1 min, it is advisable to use a UV/VIS radiation source having a power between 10 mW/cm² and 20,000 mW/cm², preferably between 30 mW/cm² and 15,000 mW/cm², and a radiation dose between 150 mJ/cm² and 20,000 mJ/cm², preferably between 500 mJ/cm² and 10,000 mJ/cm². Within the scope of these power and dose values, it is possible to achieve area-specific irradiation times between a maximum of 2000 s/cm² and a minimum of 8 ms/cm².

Preferably, the method proposed finds use in the production of objects that are elastomer parts, especially silicone elastomer parts. For the production of the elastomer part, preference is given to using one of the print materials proposed above. Elastomers, especially silicone elastomers, place specific demands on the 3D printing process since these materials, by contrast, for example, with thermoplastics, are elastic and can be deformed during the production of the object. Moreover, the uncrosslinked materials are free-flowing until they have cured.

The invention also relates to an elastomer part, especially a silicone elastomer part, produced by the process proposed. The elastomer part is preferably constructed using one of the print materials described above. The elastomer part produced by the process proposed is notable for a quality which can correspond to or even exceeds the quality of elastomer parts produced by means of injection molding. At the same time, the surface can be adjusted as desired. The surface can, for example, be structured, especially given a regular structure, or may be smooth and/or completely continuous. The elastomer parts produced in accordance with the invention, owing to the reprinting of erroneously unplaced print materials, also do not nave any trapped air or gas bubbles. Thus, mechanically stressable objects with reliable physical properties can be produced, which are also suitable, for example, for medical applications. For example, it is possible to assure elasticity or smoothness properties, or isotropic optical transparency in the case of optical lenses. In addition, it is a feature of the elastomer part that its geometry is not limited by the molds used in casting methods. Thus, the elastomer part can have undercuts and/or enclosed cavities. The elastomer part is likewise free of burrs which occur in injection-molded parts especially at the separation of the mold halves and at the runner system.

In one embodiment, the position of the printhead is constantly determined by a position measurement and the print materials are placed by the discharge device depending an the constantly determined position of the printhead. More particularly, the relative position of the printhead to the baseplate and hence to the object to be produced is established. Preferably, for this purpose, the position is determined for each of the three spatial directions X, Y and Z. The position is determined at least for those spatial directions that are in a plane parallel to the baseplate. The print material is discharged through the discharge device in each case taking account of the actual position ascertained and not taking account of an assumed target position.

The constant determining of the position of the printhead is advantageous, for example, if the production of the object is interrupted. An interruption of this kind may become necessary, for example, in order to clean the discharge device used. For this purpose, the printhead may be brought to a safe position removed from the already partly formed object, where it may be cleaned. Any changes in position of the printhead the cleaning, for example through forces transmitted to the printhead during the cleaning, are constantly determined as before and taken into account when the printing operation is continued again. The cleaning intervals are preferably programmable, such that they can be adjusted depending on the print material processed.

A further reason for an interruption may be the triggering of a safety device. The printhead is a moving part, and there is the risk that a user of the 3D printing device will be injured if, for example, his/her hands come close to moving parts. In the prior art, therefore, emergency off switches are customary, which stop the power supply to the positioning unit used. Owing to the inertia of the printhead or owing to external forces being applied to the printhead, it can continue to move even after the power supply has been stopped, and so its actual position can differ from the last known target position. Advantageously, operation of the position measurement unit continues, such that, even when an emergency off switch is triggered, the position of the printhead is still continuously determined. For this purpose, it is preferably the case that the power supply for the position measurement unit is separated from that of the positioning unit. This enables seamless continuation of the production of the object in that the printhead, after restoration of the power supply of the positioning unit, is guided back to its target position. Advantageously, it is thus possible to complete and conserve, in particular, complex or costly 3D objects that would otherwise have been rejects after an emergency switchoff.

A further advantage of the constant determination of the actual position of the printhead is that failure to reach a defined position can be recognized. A defined target position is considered not to have been reached when the actual position of the printhead determined differs by more than a given tolerance. This can, for example, be fixed, for example in the range from 0.1 to 0.5 mm. It is likewise conceivable to define the tolerance in relation to the size of a voxel or a strand, for example based on the diameter of a virtually spherical voxel or the diameter of a strand. For example, a position can be considered to be “not reached” when the measured position does not correspond to the target position within the range of tolerance at any time within a defined period. Positions that have not been reached are, for example, communicated back to the control unit, and then the information is processed further, for example recorded and used to control the remainder of the printing operation.

In a further-preferred embodiment of the invention, in addition to the described determination of erroneously unplaced print materials using the information gained as to the geometry of the print materials, these erroneously unplaced print materials can also be determined via the constant measurement of position. If a defined position is not reached by the printhead, it is not possible for print material to be discharged as intended at the defined position. This is likewise manifested in the object as erroneously unplaced print material. If an erroneously unplaced print material is recognized in this way, this is preferably recorded together with the position at which the print material should be placed. In order to reprint the unplaced print materials, the printhead is moved back to the recorded position and the discharge, as already described, is preferably repeated prior to curing of the print materials.

3D Printing Method

In order to construct an object with the print materials discharged, the print materials are deposited on the baseplate according to a defined scheme, forming a first material layer. After the first material layer has been formed, for example, the distance between the discharge device and the baseplate is increased and the next material layer is deployed. These are followed by further material layers, each of which is deposited according to a defined scheme, until the desired object has been completed.

The print materials are applied according to a scheme derived from a template. The template has generally been designed with CAD (Computer-Aided Design) software or is created by three-dimensional scanning of an article. For the derivation of the scheme for the material discharge, the software typically calculates horizontal sections of the template, with each of these sections corresponding to a material layer. Subsequently, a calculation is made as to how the print materials have to be positioned in the respective layer. What is taken into account here is whether the print materials are discharged in the form of voxels, in the form of strands, or in a combination of voxels and strands.

If appropriate, the placing of support material is also allowed for in the derivation of the scheme. The placing of support material may be necessary when the object to be produced is to have cavities, undercuts, overhanging, self-supporting or thin-walled parts, since the print materials cannot be placed free-floating in space. The support material fills cavities during the printing process and serves as a basis or scaffold in order to be able to place and cure the print materials thereon. After the printing process has ended, the support material is removed again and clears the cavities, undercuts and overhanging, self-supporting or thin-walled parts of the object. In general, the support material used is a material different from the material of the print materials. Depending on the geometry of the object, the necessary shape of the support material is calculated. In the calculation of the shape of the support material, it is possible to use various strategies in order, for example, to use a minimum amount of support material or to increase the trueness to scale of the product. The support material discharged is preferably geometrically measured, just like the further print materials described above. For this purpose, a separate device may be provided.

In the derivation of the scheme for the material discharge from the horizontal sections, it is possible to use various movement strategies, where the choice of the movement strategy can also affect the properties of the object produced. In the case of discharge in the form of voxels, it is possible to use, for example, a dual movement strategy, a xing (“crossing”) movement strategy or a border movement strategy.

In the dual movement strategy, the printhead is moved back and forth line by line in a selected main print direction in relation to the baseplate, and print material is dosed line by line. After each line, the printhead is moved further by one line width at right angles to the printed line, then the discharge of the print material is repeated. The process is similar to the printing of a conventional inkjet printer.

The xing movement strategy corresponds very substantially to the dual movement strategy. By contrast to the dual movement strategy, the main printing direction is rotated by 90° after every or after every nth material layer (where n is a natural number). This leads to more exact compliance with dimensional stability of the object because the rotation of the material layers ensures a homogeneous distribution of the print material.

Advantages of the dual movement strategy and also of the xing movement strategy are that, for example, high accuracy of the edges of the object can be achieved.

In the border movement strategy, for each material layer, first the outer circumference of the object is produced and subsequently the enclosed area is filled in, for example by means of the dual movement strategy or the xing movement strategy. In the border movement strategy, the outer circumference of the layer to be printed can be generated with smaller voxels in higher resolution than the interior of the layer to be printed or the internal volume of the object. In the interior of the object to be produced, it is possible to use larger voxels for filling, without the associated lower resolution having any effect on the accuracy of the geometric dimensions or on the surface quality of the object to be produced.

The movement strategies named are basic patterns. They can also be combined and varied within the scope of the same object if it seems appropriate. Mixed forms between the different movement strategies are also conceivable.

It the print materials take the form of voxels, in addition to the choice of movement strategy, it is also possible to include a voxel offset in the creation of the scheme for the material discharge. In this case, the voxels are not rigidly aligned in an orthogonal pattern within a layer, but may be placed offset to one another. For example, in the case of approximately spherical voxels, every second line can be placed offset by half a voxel diameter. This can reduce the line separation compared to an orthogonal grid. The voxels are placed more densely and there is a rise in the surface quality. The offset of the voxels can decrease the edge sharpness.

Additionally or alternatively to offset placing of the voxels in the plane of a material layer, it is possible to dispose the voxels of two adjacent planes offset from one another.

If the discharge device has been set up to place voxels of different size, it is additionally possible, especially in the region of the edges of the object, to vary the size of the print materials placed in order to achieve a higher edge sharpness. Preferably, the site at which a voxel is placed and the size thereof are chosen such that the edges of the object are reproduced with maximum exactness. For example, in the region of an edge, multiple smaller voxels are positioned rather than an individual voxel. The achievable edge sharpness and/or surface quality is increased as a result.

The properties of the object to be produced can be affected by appropriate choice of the parameters used in the placing of the print materials, especially the parameters of the discharge device. Examples of properties of the object that can be influenced are edge sharpness, surface quality and dimensional stability. The properties of the object to be produced are determined by the configuration of the discharge device before commencement of printing.

Edge sharpness is understood to mean the sharpness of the delimitation of a region belonging to the object where print materials are to be placed with respect to a region outside the object where no print materials are to be placed. The more abrupt the transition, the higher the edge sharpness. Typically, the edge sharpness improves when the size of the voxels or the diameter of the strands is reduced. Conversely, the edge sharpness falls when the size of the voxels or the diameter of the strands is increased.

The surface quality is understood to mean the smoothness of a surface. A surface of high quality is continuous and smooth, free of depressions and bulges, for example with a surface roughness Ra of <4 μm. Surfaces of this kind are ideally achieved, for example, by injection molding.

Dimensional stability is understood to mean the trueness to scale of the geometric dimensions of the object, i.e. that they have only small deviations, if any, from the dimensions of the template.

Some jetting or dispenser operating parameters can, in the context of the invention, also be varied during printing, for example in each case after one or more material layers or even from voxel to voxel, from strand to strand, from voxel to strand or from strand to voxel, for example even depending on the known actual positions of the voxels and strands already placed, which enables readjustment of the properties of the object to be produced.

Preferably, in the case of print materials in the form of voxels, the edge sharpness of the object is set or readjusted by adjustment of the voxel size and/or the surface quality of the object is set by adjustment of the voxel offset and/or the dimensional stability is set by adjustment of the movement strategy of the 3D printing device. The voxel size can be varied by the configuration of the jetting parameters.

Preferably, in the case of print materials in the form of strands, the edge sharpness and the surface quality of the object are set or readjusted by adjustment of a volume flow rate and/or the dimensional stability is set by adjustment of the movement strategy of the discharge device. The volume flow rate can be varied as described by the configuration of the dispenser parameters.

The volume flow rate refers to the volume of the print material discharged per unit time. In the placing of strands, the printhead moves with the discharge device during the discharge of the strand relative to the baseplate or the object. The shape of the strand placed an the baseplate or the object is dependent on the volume flow rate and an instantaneous speed of the printhead, and on the distance from the baseplate, or, in further embodiments, on a penetration depth of the nozzle into the last layer. It is advantageous, for placing of a defined amount of print material, to match the volume flow rate to the instantaneous speed in order that the shape of the strand placed corresponds to the desired shape.

The instantaneous speed can be calculated, for example, from the constantly determined position of the printhead, in that the position of the printhead is determined at two junctures, and the difference between the positions determined is formed and divided by the time elapsed between the two junctures.

For the implementation of the curing, a curing strategy is used. Preferably, curing of the print materials follows the placing of a layer of print materials or the placing of multiple layers of print materials, or is effected directly during printing.

Curing of the print materials directly during printing is referred to as a direct curing strategy. If print materials curable by UV/VIS radiation are used, by comparison with other curing strategies, the UV/VIS source is active for a very long period, and so it is possible to work with very much lower intensity, which leads to slower crosslinking through the object. This limits the heating of the object and leads to objects that are true to scale since no expansion of the object occurs owing to temperature peaks.

In the per layer curing strategy, the placing of every complete material layer is followed by the radiation-induced crosslinking of the material layer placed. During this operation, the freshly printed layer becomes bonded to the cured printed layer beneath. The curing does not follow immediately after the placing of a print material, and so the print materials have time to relax before the curing. What is meant thereby is that the print materials can merge with one another, which achieves a smoother surface than in the direct curing strategy.

In the nth layer curing strategy, the procedure is similar to that in the per layer curing strategy, except that the curing is undertaken only after the placing of n material layers where n is a natural number. The time available for the relaxing of the print materials is increased further, which further improves the surface quality. Owing to the flow of the print materials, however, thee can be a decrease in the edge sharpness achievable.

In a preferred embodiment, the curing strategy is matched to the reprinting of unplaced print materials. For example, the printing of a material layer may be followed in each case by the reprinting of the erroneously unplaced print materials before the crosslinking of the material layer placed is effected by the per layer curing strategy or nth layer curing strategy.

3D Printing Device

A further aspect of the invention is that of providing a 3D printing device for production of an object by the 3D printing method. The 3D printing device has at least one printhead having at least one discharge device, wherein the discharge device has a control unit in order to place print materials at target positions in order to additively manufacture the object.

The 3D printing device is designed and/or configured to execute one of the methods described herein. Accordingly, features described in the context of the methods are disclosed correspondingly for the 3D printing device and, conversely, the features described in the context of the 3D printing device are correspondingly disclosed for the method.

According to the invention, the 3D printing device has at least one optical measurement unit arranged and set up such that print materials exiting from the discharge device can be geometrically measured in the region of the discharge device.

The optical measurement device may be a camera. Preferably, with the aid of the camera, imaging is possible in the order of magnitude of the jetting frequency in the case of voxel printing. More particularly, it is possible to use high-speed cameras, the images from which are evaluated with appropriate image processing software.

Alternatively and preferably, the optical measurement unit comprises a light barrier with an optical emitter and an optical receiver. Preference is given here to using light-reduced laser micrometers.

In a preferred embodiment, the emitter and the receiver are disposed by means of suitable holders on a movement axis of the printhead. Alternatively, securing directly on the printhead or on the discharge device is possible. The optical measurement device may be mounted at a fixed distance from the exit opening of the discharge device or at a variable distance by means of an adjustable holder.

In a further embodiment, the optical measurement device comprises at least one optical fiber. In each case, typically one optical fiber is provided for each of the emitter and the receiver. In reflected light operation, it may be the case that just a single optical fiber is connected to the discharge device, in which case an emitter/receiver unit is used.

The optical fiber is preferably arranged such that its light exit area is either integrated into the discharge device or is secured to the discharge device by means, of an attachment, such that the exiting light beam, for example a laser beam, is directly integrated at the exit of the discharge device or in the droplet or strand channel. Emitter and receiver may, in this embodiment, be arranged independently of the printhead in the 3D printing device. This variant has the advantage that the optical fiber can be disposed at a very small distance from the droplet or, in the integrated variant, at zero distance, such that the light beam is exposed to a minor level of external influences, for example particles flying around. It is further advantageous that no heavy components have to be disposed on the printhead or on the movement axis, which also have to be accelerated during the 3D printing.

In a preferred embodiment, at least one weight sensor is assigned to the baseplate. The weight can be measured, for example, electronically at the contact points of the baseplate. The weight sensors can constantly determine and record the mass, or increase in mass, of the printed object. Even the mass of a single print voxel can be determined by measuring the increase in weight of the printed body on the baseplate. In combination with the droplet recognition and monitoring of the print material that has exited, recognition of any foreign body contamination of the print material is achievable. If foreign body contamination is detected, automated messages or actions are correspondingly generated and executed.

If an abrupt increase in weight is detected on the baseplate, for example above a particular weight limit such as 1 kg, the printing process may immediately be interrupted. This is a personnel protection measure in the event of unauthorized encroachment into the printing region. The printhead is not moved any further here, and alarm messages are issued.

In a preferred embodiment, the 3D printing device also has a position measurement unit, by means of which the position of the printhead can be determined constantly, wherein the position measurement unit is connected to a control unit of the discharge device, and wherein the discharge device is set up to place the print materials depending on the constantly determined position of the printhead.

The 3D printing device further comprises a baseplate on which the object is built up by discharge of print material from the discharge device of the printhead. The baseplate and the printhead here are moved relative to one another, with relative movements being possible in all three spatial directions X, Y and Z. For this purpose, for example, the printhead may be arranged such that it is movable in X and Y direction, and the baseplate can be arranged such that it is movable in Z direction. Further configurations are also conceivable here; for example, the baseplate may be arranged so as to be movable in Y direction and the printhead may be arranged so as to be movable in X and Z direction. Alternatively or additionally, the baseplate and/or the printhead may be configured so as to be pivotable, such that any desired spatial arrangements are possible.

It is possible to provide multiple discharge devices, including those that are technically different, for various print materials in the 3D printing device.

The respective discharge device has a discharge axis which defines the direction in which material is discharged from the discharge device. Typically, the discharge axis is oriented with reference to the baseplate such that it is at right angles to the baseplate. Optionally, the 3D printing device may be configured such that the alignment of discharge axis can also be altered relative to the baseplate.

For release of individual droplets, the discharge device may comprise one or more nozzles which emit liquid droplets of print material in the direction of the baseplate, similarly to the manner of the nozzles of an inkjet printer. Therefore, these nozzles are also referred to as jetting nozzles. Various embodiments are known to those skilled in the art. The jetting nozzles are set up such that they release a droplet in a controlled manner only on demand. In preferred embodiments of the jetting nozzles, on discharge of the print material, the volume of the droplet can be affected, such that droplets of different size can be generated.

It is possible to provide, for example, a heating element in the jetting nozzle, with which the print material is heated, and a droplet of a vapor bubble that arises is driven out of the jetting nozzle; this is known as a bubblejet.

A further option is the arrangement of a piezo element which deforms owing to an electrical voltage and, as a result, can eject a droplet from a jetting nozzle. Inkjet printing methods of this kind are known in principle to the person skilled in the art from conventional printing and from what is called 3D printing, in which three-dimensional articles are built up layer by layer from a photopolymerizable ink. Printheads of this kind, as used in inkjet printing or in multijet 3D printing, can typically dose low-viscosity printing inks or print materials, for example those with viscosities below 50 mPa·s.

In the printheads in the method of the invention, preference is given to using discharge devices based on jet valves with piezo elements. These enable the discharge both of low-viscosity materials, where droplet volumes for droplets of a few picoliters (pL) (2 pL correspond to a droplet diameter of about 0.035 μm) can be achieved, and of moderate- and high-viscosity materials such as, the silicone rubber materials, where preference is given to piezo printheads with a nozzle diameter between 50 and 500 μm and droplet volumes in the nanoliter range (1 to 100 nL) can be generated. With low-viscosity materials (<100 mPa·s), these printheads can deposit droplets with very high dosage frequency (about 1-30 kHz), whereas, with higher-viscosity materials (>100 Pa·s), depending on the rheological properties (shear-thinning characteristics), dosage frequencies of up to about 500 Hz can be achieved. Suitable jetting nozzles are described, for example, in DE 10 2011 108 799 A1.

For release of strands of print material, the print material is expressed as a strand through a nozzle by means of pressurization of a reservoir vessel, for example from a cartridge, syringe or vat, and selectively deposited on the baseplate to form the object. Discharge device of this kind are referred to in the context of this description as dispensers. The pressure can be built up, for example, by means of air pressure or by mechanical means, for example by a small extruder, by means of a piston pump or by means of an eccentric screw. Various embodiments are known to those skilled in the art.

The discharge device has a control unit. The control unit controls the placing of print materials by the discharge device. For placing of the print materials, the control unit can make use of the target position derived from the scheme and preferably additionally of the constantly determined actual position and further preferably additionally also of further input parameters, for example the current speed of the printhead. The control unit may, for example, comprise a microcontroller connected to the position measurement unit and the discharge device. The control unit may be executed as a separate unit or else in combination with a machine control system of the 3D printing device. The machine control system may likewise comprise a microcontroller, in which case this is connected to a positioning unit.

The positioning unit is set up to position the printhead relative to a baseplate, where the relative position is adjustable at least along the three spatial axes X, Y and Z, and possibly also rotatable. The positioning unit comprises at least one motor, typically with at least one separate motor provided for every adjustable spatial axis. The motor is executed, for example, as an electric motor, especially as a stepper motor.

The discharge device has, a jetting nozzle actuated by the control unit or a dispenser. In one embodiment, the 3D printing device has multiple discharge devices configured with assignment to a printhead. The printhead here may have multiple different discharge devices, for example one or more jetting nozzles and one or more dispensers. In this case, for example, the print materials can be rapidly placed in the interior of the object by means of the dispenser(s) and the surface of the object can be produced in high quality with the jetting nozzle(s). Alternatively, it is conceivable that the printhead comprises multiple equivalent discharge devices. In this way, for example, multiple objects can simultaneously be additively produced, or it is possible to work with multiple discharge devices in parallel on the construction of a single object. In both cases, the printing time required overall is reduced.

In the case of a jetting novel as discharge device, the control unit defines when the jetting nozzle discharges a voxel. In addition, the control unit may define the size of the voxel.

In the case of a dispenser as discharge device, the control unit defines when the dispenser commences with the discharge of print material in the form of a strand and when the discharge is ended. In addition, the volume flow rate, i.e. how much print material is discharged within what time, may be defined by the control unit.

If support material is used, the printhead may have one or more further discharge devices for the support material. Alternatively or additionally, it is also possible for a further printhead with an appropriate discharge device to be provided for the discharge of support material.

The position measurement unit is set up to constantly determine the position of the printhead. For this purpose, the position measurement unit may undertake measurements of the position of the printhead at a defined rate and transmit them to the control unit.

The position measurement unit is preferably set up to undertake a measurement of the position with reference to every axis or spatial direction adjustable by the positioning unit.

The position measurement unit is at least set up to determine the position of the printhead within a plane parallel to the baseplate. It is preferably set up to determine the position of the printhead in space.

The position measurement unit preferably has at least one step counter in the motor, rotary encoder, optical scale, especially a glass scale, GPS sensor, radar sensor, ultrasound sensor, LIDAR sensor and/or at least one light barrier. The step counter in the motor may especially be configured as a contactless switch, for example as a magnetic sensor, especially a Hall sensor.

The 3D printing device preferably additionally has a main controller containing a template or a computer model of the object to be printed, where the main controller and the control unit of the discharge device are set up for bidirectional communication with one another.

The main controller may be executed, for example, as a computer which communicates with the control unit, for example via a data network, for example ethernet or WLAN, or via a connection, for example a serial connection or USB.

The computer model may be recorded in the main controller in any data format. Standard data formats include, for example, STL, OBJ, CLI/SLC, PLY, VRML, AMF, STEP, IDES. In the execution of the method described, the main controller produces virtual horizontal slices through the model (called slicing). These horizontal sections are subsequently used to calculate a scheme which states how the print materials have to be positioned for additive construction of the object. What is taken into account here is whether the print materials are discharged in the form of voxels, in the form of strands, or in the form of a combination of voxels and strands. If the shape of the object requires the placing of support material, the main controller is preferably set up to generate a scheme for placing of support material as well. The calculation and placing of support material can also be effected in a decoupled manner.

During the production of the object, the main controller and control unit communicate with one another, such that the main controller can update the scheme depending on the exiting print materials detected, and especially on the erroneously unplaced print materials detected, the determined position of the printhead and optionally further determined parameters. The main controller can likewise receive messages about errors that occur and/or about erroneously unplaced print materials, which can be taken into account correspondingly.

The direct feedback of the position of the printhead to the main controller makes it possible for the main controller to directly influence the movement pathways of the printhead. For example, it is possible to accelerate and slow down the printhead outside the object, such that the printhead moves at constant speed relative to the object during the actual printing operation in which print materials are placed. Especially at high speeds, it is thus possible to avoid or reduce resonances or vibrations, which leads to a higher quality of the object. The acceleration outside the object leads to shorter dead times in idle runs, which reduces the time required for the printing and hence in turn the printing costs per object.

If print materials which cure under UV/VIS are used, the 3D print device preferably has a UV/VIS source. In the case of location-selective exposure, the UV/VIS source is arranged so as to be movable relative to the baseplate and illuminates only selected regions of the object. In the case of low-area exposure, the UV/VIS source, in one variant, is configured such that the entire object or an entire material layer of the object is exposed all at once. In a preferred variant, the UV/VIS source is designed such that its light intensity or its energy can be variably adjusted and that the UV/VIS source exposes just a subregion of the object at any time, it being possible to move the UV/VIS source relative to the object in such a way that the entire object can be exposed with the UV/VIS light, optionally in different intensity. For example, the UV/VIS source, for this purpose, is configured as a UV/VIS LED bar and is moved relative to the object, or over the printed object.

Preferably, the 3D printing device also comprises a cleaning station which enables the automatic cleaning of the discharge device of the printhead. Owing to the constant determination of the position of the printhead, cleaning can also be undertaken during the production of an object. For this purpose, the printing is interrupted and the printhead is moved to the cleaning station. After conducting the cleaning operation, the printhead is guided precisely to the next position at which a print material is to be placed and the printing operation is continued.

The figures show working examples of the invention, although the figures show the subject matter of the invention merely in schematic form. The working examples shown and described hereinafter with reference to the figures should not be regarded as being restrictive in respect of the subject matter of the invention. A multitude of modifications possible within the scope of the claims will be apparent to the person skilled in the art.

The figures show:

FIG. 1 illustrates a schematic diagram of a working example of a method of actuating a discharge device,

FIG. 2 illustrates a schematic setup of a 3D printing device in one embodiment of the invention,

FIG. 3 illustrates a side view of a strand discharge and a strand measurement in one embodiment of the invention,

FIG. 4 illustrates a top view of the strand measurement as per FIG. 3,

FIG. 5 a side view of a droplet measurement in one embodiment of the invention,

FIG. 6 illustrates a side view of a printhead and optical measurement unit arranged in a holder in one embodiment of the invention,

FIG. 7 illustrates a side view of a printhead with an emitter and receiver in one embodiment of the invention, and

FIG. 8 illustrates a side view of a printhead with an emitter/receiver unit in one embodiment of the invention.

In the description of the working examples of the invention which follows, identical or similar components and elements are identified by identical or similar reference numerals, in which case repeated description of these components or elements is dispensed with in individual cases.

FIG. 1 shows a schematic of the procedure of a working example of the method of the invention. A template for an object to be produced is recorded in the main controller 12. The main controller 12 uses this to determine a scheme which describes the sites at which print materials have to be placed to produce the object. These sites are target positions. These target positions are transmitted to a control unit 14. The control unit 14 is connected to a machine control system 16. By means of the machine control system 16, a positioning unit 18 is actuated in order to move a printhead 22 to the target position. Alternatively, the machine control system 16 may have a direct connection to the main controller 12 (not shown).

The movement of the printhead 22 is monitored by means of a position measurement unit 20. The position measurement unit 20 and positioning unit 18/printhead 22 are coupled to one another, especially by a mechanical connection, in such a way that any change in position of the printhead 22 is determined by the position measurement unit 20. The position measurement unit 20 communicates the determined position of the printhead 22 back to the control unit 14.

The printhead 22 comprises a discharge device 24 set up to place print materials for construction of the object. The control unit 14 is connected to the discharge device 24 and controls the discharge of the print materials. It is the case here that the control unit 14 actuates the discharge device 24 depending on the position of the printhead 22 determined constantly by means of the position measurement unit 20. Thus, print materials are placed taking account of the actually determined actual position and not, for instance, under the assumption that the printhead 22 is at the target position.

It may also be the case that the determined positions of the printhead 22 are fed back to the main control system 12 by the control unit 14 via a bidirectional connection. This puts the main control system 12 in a position to plan the further placing of the print materials depending on the determined positions at which print materials have already been placed.

Moreover, as shown, the position measurement unit 20 may additionally communicate the constantly determined position of the printhead 22 to the machine control system 16. In one variant of the method, the latter can generate an error message as a result of this feedback and transmit it to the control unit 14 if the printhead 22 was unable to reach a target position. It is likewise possible to implement a closed-loop control circuit in order to actuate the positioning unit 18 in such a way that the deviation of the position of the printhead 22 determined by the position measurement unit 20 from the target position is minimized. For this purpose, the actual position of the printhead 22 can be readjusted constantly to the target position by means of the positioning unit 18, which is also referred to in the context of the present disclosure as readjustment for taking up an exact position.

FIG. 2 shows a schematic of a 3D printing device 10. The 3D printing device 10 comprises the main control system 12 which contains the template for an object 40 to be produced and is connected to the control unit 14. The 3D printing device 10 further comprises a base plate 30 on which the object 40 is built up additively by the placing of print materials 42.

For placing of the print materials 42, the printhead 22 in the working example shown comprises two discharge devices 24. A discharge device 24 is executed as a jetting nozzle 28. The jetting nozzle 28 places the print materials 42 in the form of individual droplets or voxels 44. The other discharge device 24 is configured as a dispenser 26 and places the print materials 42 in the form of strands 46.

In the example shown in FIG. 2, both the jetting nozzle 28 and the dispenser 26 are used for additive construction of the object 40, by using the jetting nozzle 28 to place voxels 44 that form the surface of the object 40 and using the dispenser 26 to place strands 46 in order to rapidly fill up the interior of the object 40.

If print material 42 which cures by action of UV/VIS radiation is used, preference is given to providing a UV/VIS light source. In the embodiment of FIG. 2, for this purpose, an LED bar 34 which emits UV/VIS light in a location-selective manner is provided. In order to be able to cover the area of the baseplate 30 with UV/VIS light, the LED bar 34 is designed so as to be movable. In the case of thermally curing print materials 42, as an alternative, preference is given to providing an IR light source set up for location-selective heating of the print materials 42. For this purpose, the IR light source may especially be secured to the printhead 22. Alternatively, a heatable space can be used for curing.

For positioning of the printhead 22, i.e. relative to the baseplate 30, the 3D printing device 10 also comprises three positioning units 18, where each of these positioning units 18 enables movement in one of the three spatial axes X, Y and Z. For this purpose, each of the positioning units 18 is connected to an axis 32 along which movement is enabled. In the working example shown in FIG. 2, for this purpose, one of the positioning units 18 is assigned to the baseplate 30 and enables movement of the base plate 30 in the spatial direction designated “Z”. Two further positioning units 18 are assigned to the printhead 22 and enable the printhead 22 to move in the spatial directions designated “X” and “Y”. All three positioning units 18 together enable positioning of the printhead 22 or in any of the three spatial directions relative to the baseplate 30. The positioning units 18 are actuated by the machine control system 16 which communicates in turn with the control unit 14.

To determine the position of the printhead 22, the 3D printing device 10 has three position measurement units 20. The position measurement units 20 are each assigned to one of the three spatial directions X, Y and Z, and detect the movement of the printhead 22 or of the baseplate 30, such that the relative position of the printhead 22 to the baseplate 30 is determined constantly. The position measurement units 20 are connected to the control unit 14. In addition, a connection to the machine control system 16 may be provided.

FIG. 3 shows a side view of a strand discharge and a strand measurement in one embodiment of the invention, for example using the 3D printing device 10 described with reference to FIG. 2.

The discharge device 24 here comprises, by way of example, the dispenser 26. The dispenser 26 deposits the print material 42 in the form of strands 46 on the baseplate 30. The deposited strands 46 result in layers 49 which form the object 40. The dispenser 26 is arranged at a discharge height H above the object 40, which, after completion of a layer 49, is readjusted again.

Also shown is an optical measurement unit 50 of the invention, in the form here of a light barrier. The optical measurement unit 50 comprises an emitter 52 and a receiver 54 arranged opposite one another. The emitter 52 emits a light beam 56 which is received by the receiver 54. The measurement is effected in transmitted light operation. The receiver 54 receives the light emitted by the emitter 52, which has been reduced by corresponding proportions as a result of absorption and reflection at the print material 42.

The optical measurement unit 50 enables measurement of the exiting strand 46 in the region of the discharge device 24. For this purpose, the light beam 56 is positioned essentially perpendicularly to a discharge axis 35 of the discharge device 24. The perpendicular arrangement of the light beam 56 allows the cross section of the discharged strand 46 to be geometrically measured in the region of the discharge device 24.

Also shown in schematic form is a weight sensor 66 assigned to the baseplate 30. The weight is measured electronically at contact points of the baseplate 30. The weight sensor 66 can constantly determine and record the increase in mass of the printed object 40.

FIG. 4 is a top view of the strand measurement as per FIG. 3.

The emitter 52 emits the light beam 56, and it is received by the receiver 54. The strand 46 present in the light beam 56 gives rise to a shadow 57 behind it. The size of the shadow 57 is dependent on the geometry of the strand 46, more specifically on a cross-sectional area of the strand 46 in the working example shown. Two cross sections of strands 46 are shown, namely an optimally cylindrical strand cross section 58 and a deformed strand cross section 60. The movement of the printhead 22 in the course of printing results in deformation of the strand 46, which is apparent, for example, in FIG. 3. Alternatively, the deformation of the strand 46 arises in the case of too low a discharge height H. By the measurement of the shadow 57 or of the incident luminance at the receiver 54, it is possible to determine whether there is a tolerable degree of deformation or not. According to the invention, this is optionally corrected, for example by increasing the discharge height H or by slowing the printing speed.

The width b of the light beam 56 with which the measurement is made is matched to the strand diameter d such that corresponding meaningful measurements are possible.

FIG. 5 shows a side view of a droplet measurement in one embodiment of the invention. A droplet 48 is shown, which has been discharged, for example, by the 3D printing device 10 shown in FIG. 2. The direction of movement v of the droplet is indicated by an arrow. It corresponds essentially to the discharge axis 35, with a component perpendicular thereto arising as a result of movement of the printhead 22 in the course of printing.

The droplet 48 comprises a spherical section 45 and a tipped section 47. The spherical section 45 is assigned a maximum diameter D. In the presence of stringing, the tipped section 47 is highly extended. During flight, i.e. after it has become detached from the jetting nozzle 28 and before it has hit the baseplate 30 or the already printed part of the object 40 on which the droplet 48 forms a voxel 44, the droplet 48 passes through the light beam 56.

The cross section of the light beam 56 shown is also referred to as measurement window in the present disclosure. It has a width b and a height h which are matched to the maximum diameter D. In the working example shown, the width b of the light beam 56 is chosen to be somewhat greater than the diameter of the droplet 48. The height h of the light beam 56 is chosen to be about one fifth of the width b of the light beam 56. The light beam shown is therefore in effectively one-dimensional form. Nevertheless, it is possible to determine the droplet geometry at the receiver at least such that, for determination of unplaced print materials, the existence of droplets 48 in the light beam 56 can be detected and that stringing can be detected. In alternative embodiments, the height h chosen for the light beam 56 is greater.

As apparent from FIGS. 4 and 5, it may be advantageous to determine the cross section of the droplet 48 or the strand 46 by arranging multiple emitters 52 and multiple receivers 54 around the droplet 48 or strand 46, for example two or three emitters 52 and two or three receivers 54. In this way, the deviation is detectable not just in one dimension, but in two or three dimensions, such that conclusions regarding the rotational symmetry of the droplet 48 or of the strand 46 are possible. For determination of unplaced print materials and stringing, just one emitter 52 and one receiver 54 are sufficient, since all that matters is the existence and amount of print material 42 in the light beam 56.

FIG. 6 shows a side view of a printhead 22 and an optical measurement unit 50 arranged in a holder 62 in one embodiment of the invention. The printhead is arranged so as to be movable on a movement axis 36. In the 3D printing device 10 described with reference to FIG. 2, the movement axis 36 corresponds to the y axis 32 shown therein. The baseplate 30 is movable independently from the printhead 22 in the Z axis.

The optical measurement unit 50 again comprises the emitter 52 and the receiver 54, which are positioned such that they can measure print materials 42 that have exited close to the discharge device 24. The emitter 52 and the receiver 54 are secured, on holders 62, the holders 62 being connected in a fixed manner to the movement axis 36. Thus, if the printhead 22 moves together with the movement axis 36 along an axis at right angles to the movement axis 36, the emitter 52 and the receiver 54 are correspondingly moved as well.

FIG. 7 shows a side view of a printhead 22 with an emitter 52 and a receiver 54 in one embodiment of the invention. The emitter 52 and the receiver 54 are each brought closer to the discharge device 24 by means of an optical fiber 64. The measurement takes place in transmitted light operation. This embodiment has the advantage over the embodiment described with reference to FIG. 6 that the emitter 52 and the receiver 54 can in principle be positioned anywhere on the 3D printing device 10 and need not necessarily be carried along in the case of movement of the printhead 22. The light exit area of the optical fiber 64 here is in the immediate vicinity of the discharge opening of the discharge device 24; for example, it is either integrated into the droplet or strand channel of the discharge device 24 or it is secured by means of an attachment (not shown) on the discharge device 24.

FIG. 8 shows a side view of a printhead 22 with an emitter/receiver unit 68 in one embodiment of the invention. The optical measurement unit 50 is formed here from the emitter/receiver unit 68 and comprises a single optical fiber 64 which approaches the discharge device 24. The measurement is effected in reflected light operation. 

1.-14. (canceled)
 15. A method for producing an object using a 3D printing device having at least one printhead with at least one discharge device, wherein the discharge device is configured to place print materials at target positions in order to additively manufacture the object, comprising: detecting and/or geometrically measuring print materials and/or support materials exiting from the discharge device by means of at least one luminance measurement in transmitted light operation or in reflected light operation in the region of the discharge device while the support materials and/or print materials are exiting or have exited the discharge device and before they have been placed.
 16. The method of claim 15, wherein the weight of the print materials that have exited from the discharge device is also measured.
 17. The method of claim 15, wherein the measured geometry and optionally the measured weight of the print materials that have exited from the discharge device are used to detect print errors, especially erroneously unplaced print materials, ingress of foreign bodies and/or stringing.
 18. The method of claim 17, wherein at least one print error is an erroneously unplaced print material, ingress of a foreign body, or stringing.
 19. The method of claim 15, wherein erroneously unplaced print materials detected are reprinted.
 20. The method of claim 19, wherein reprinting takes place prior to a method step of curing.
 21. The method of claim 15, wherein on the basis of a measured geometry of the print materials that have exited from the discharge device, cleaning of the printhead is triggered and/or maintenance advice is issued.
 22. The method of claim 15, wherein the geometry of the print materials exiting from the discharge device is under closed-loop control by means of one or more operating parameters of the 3D printing device, at least one parameter selected from the group consisting of: a movement speed of the printhead; a discharge height (H) above a baseplate on which the object is additively manufactured; a physical pressure that prevails in the discharge device, to which the print material is subject; a physical supply pressure in the material reservoir; in the case of voxels, a print frequency, a ram advance speed, a ram withdrawal speed, opening time of the jetting valve, and the ram stroke, and in the case of strands, a flow rate, a feeder speed and a material retraction characteristic at the end of a line.
 23. The method of claim 22, wherein the luminance measurement comprises an evaluation of a comparison of a signal received over time with a reference signal, where the reference signal is recorded in a calibration of the 3D printing device or at the start of the printing of the object or of a layer of the object.
 24. The method of claim 15, wherein the object is an elastomer part.
 25. The method of claim 24, wherein the elastomer comprises a silicone elastomer.
 26. The method of claim 15, wherein the actual position of the printhead is constantly determined by a position measurement unit and the print materials are placed by the discharge device depending on the constantly determined position of the printhead.
 27. A 3D printing device for manufacture of an object by a 3D printing method, comprising: at least one printhead with at least one discharge device, where the discharge device has a control unit to control placement of print materials at target positions in order to additively manufacture the object, at least one optical measurement unit positioned and configured such that print materials exiting from the discharge device are detected and/or geometrically measured by means of at least one luminance measurement in transmitted light operation or in reflected light operation in the region of the discharge device as an optical measurement unit while print materials are exiting or have exited and before the print materials have been placed.
 28. The 3D printing device of claim 27, wherein the optical measurement unit is arranged in an axis of movement of the printhead.
 29. The 3D printing device of claim 27, wherein the optical measurement unit comprises at least one optical fiber, the light exit area of which is integrated into the discharge device or is secured on the discharge device by means of an attachment.
 30. The 3D printing device of claim 27, comprising at least one baseplate on which the object is to be additively manufactured, and a weight sensor assigned to the baseplate.
 31. The 3D printing device of claim 27, wherein the 3D printing device has a position measurement unit by means of which the position of the printhead is constantly determined, wherein the position measurement unit is connected to the control unit of the discharge device, and wherein the discharge device is configured to place the print materials as a function of the constantly determined position of the printhead. 